How TV Aerials Work

Aerial Installations
Television waves travel in straight lines rather like light rays and don’t bend much around obstacles. So, wherever you live, your receiving aerial should be outdoors, as high as possible, and in the clear, so that it gets the best direct signal. It is extremely important to use a good quality aerial from a reputable manufacturer to avoid the risk of poor reception. As television reception can vary dramatically over distances of a few feet or even a few inches, the precise location of your aerial can have a big influence on the quality of your picture. Outdoor aerials do not last indefinitely. If reception begins to deteriorate, it could be due to corrosion of the cable connections inside the aerial junction box. The aerial may even have been knocked off alignment or broken by strong winds. Inspect the aerial for mechanical damage, and check the cable downlead to make sure it has not deteriorated or been broken. Sunlight can cause it to become brittle, while the ingress of water causes a large loss of signal to the TV set. Aerials and downleads may need to be replaced more often in exposed, coastal, or industrial areas. 

     A bad installation                   An example of a good aerial installation

No matter how close you may live to the transmitting station, you should use an outside aerial.

Standing Wave Patterns
In addition to the direct signal from the transmitting station, your aerial can receive signals reflected from the nearby ground or other surfaces. These signals can add or subtract from the direct signal, giving stronger or weaker signals at your aerial. Reception may be poor, with "snowy" or "speckled" pictures because the aerial is positioned at or near a minimum signal point. In general, the patterns change over only a foot or two and are more pronounced on the highest frequency channels. Roof materials such as tiles with metal trace elements, wire netting, and other metallic roofs can cause reception problems, so the aerial should be three or more feet above and well clear of the roof.

Causes of Interference

Reflections or "ghosting"
Ghosting on a TV picture is caused by reflected signals from hills, tall buildings, cranes etc., arriving at your aerial a tiny fraction of a second after the direct signal from the transmitter. These delayed or ghost signals appear to the right of the main image on the screen. A good directional aerial is needed to reject the unwanted signals.

Co-Channel Interference
Sometimes during fine weather, horizontal venetian-blind type lines or even other TV pictures can interfere with reception. A good directional aerial will often minimize these effects, which are caused by distant stations being received over much greater distances than normal.

Building Work
Standing wave patterns and picture quality can alter if there is extensive building work and scaffolding nearby. Re-positioning your aerial may restore good reception, but a more directional aerial may be required. However, before spending any money on your aerial, it may be wise to wait until the building work is finished. Site owners and contractors cannot be held responsible for ruining your reception and you will probably have to foot the bill yourself.

Trees
Trees and their leaves reduce television signal strengths, and create complicated reception patterns around your aerial. Again, the wavelength of the signal determines where the resulting peaks and nulls are located. If large trees cannot be avoided between the aerial and the transmitter, reception can vary as the foliage moves in the wind and as the trees gain or lose their leaves in the spring and autumn. Even with evergreen trees, the rising sap in the spring can affect television reception, and two aerials may be necessary – one for optimum reception in the summer, and the other for the winter.

What Do I Need?

Aerial Downleads
A low loss coaxial cable downlead (impedance 75 ohms) is needed between the aerial and the receiver. The cable should be in one continuous length, with the centre conductor connections to coaxial plugs soldered to make good conduct and the aerial terminals adequately protected to avoid corrosion.

Lightning Protection
The risk of a television aerial and its coaxial cable downlead attracting a lightning discharge is extremely slight; most people do not take any special precautions. Fitting an earth conductor to the aerial support metalwork should keep any discharge outside the house. In no circumstances should an aerial lead be disconnected during a thunderstorm. Should a lightning discharge take place at that moment, serious injury or death could result.

Living in a Flat
If you live in a flat you are very likely to be receiving your TV signal from a communal system that distributes the TV signal to each flat in the building. A poor quality picture can be due to a failure in the communal system, a broken wall socket or a faulty lead from the wall socket to the TV set. Ask the neighbours if they get a good picture and if they don’t contact the Housing Office and ask to have the system checked. If the neighbours are getting a good picture then check the TV lead, a ready-made one can be purchased at any reputable TV shop. Check the socket, is it loose? If it is suspect then contact the Housing Office and have it repaired.

Install A Good Aerial
The key to good television reception is a good quality aerial, properly installed. Attention to detail and the correct interconnection of associated VCRs and satellite equipment should remove any reception problems. For more expert information on TV and radio reception, log on to the BBC website at..

               www.bbc.co.uk/reception.